1. Field of the invention
This invention relates to current sources and more particularly to a current drive source for driving a plurality of parallel connected PNP current sources.
2. Description of the Prior Art
In the utilization of integrated circuits it is common to use a plurality of individual parallel connected PNP current sources which provide selective currents to particular circuit portions of the integrated circuit. Typically the base electrodes of the PNP current sources are all driven from the output of a well known Phi/R current drive source. Phi/R current drive sources can be found in integrated operational amplifiers, regulators, comparators and many linear bipolar integrated circuits which use active current sources. For example, the MLM 339 "Quad Single-Supply Comparator", manufactured and sold by MOTOROLA, Inc., incorporates a Phi/R current drive source for driving a string of PNP current sources therefrom.
Although the Phi/R current drive source has a high output impedance, the PNP current sources driven therefrom typically have very poor output impedances. Therefore, if the power supply voltage supplied to both the Phi/R current drive source and the PNP current sources should vary from 5 to 40 volts (which is not uncommon in some applications) it would not be unusual to find that the collector currents of the PNP current sources vary by 100%. This increase in currents will cause greater power dissapation in the integrated circuit which, in turn, causes the integrated chip to heat up. Furthermore, if for instance the integrated circuit includes a comparator, the offset voltage of the comparator is caused to change as the current varies due to the power supply variation. This is undesirous to the user of such an integrated circuit.
Improvement in the output impedance in the PNP current sources can be accomplished by degenerating the emitters by including a series resistor in each. This is not often pratical because of the substantial chip area required by such resistors, or because there is not enough voltage to allow the added voltage drops in the resistors. Therefore, a need exists to provide better rejection to power supply variations without causing substantial chip area to be utilized, or further reducing the voltage which can be supplied to the PNP current sources.